Opentelemetry-go: Move mock span or trace out of internal package

[XSAM]

MockSpan and MockTracer are on the internal package that cannot be used by other repo.

It's not convenient for those third-party repos to add the unit test. Also, go-contrib have the same package named internal/trace and the functionality of the code is almost the same.

Therefore, maybe we should move MockSpan and MockTracer out of the internal package and reuse those parts of codes?

[MrAlias]

This is likely a duplicate of https://github.com/open-telemetry/opentelemetry-go/issues/872

[jmacd]

This looks like not quite a duplicate, since it's about tracing and #872 is about metrics.

[AndrewGrachov]

Mockmeter moved in
https://github.com/open-telemetry/opentelemetry-go/pull/1040/files